Just purchased ProBuilder, and right off the bat I have noticed the lack of thickness or backface on the extrude tool. As an example, I am trying to make an airplane fuselage, so I create a cylinder. But it needs to have faces on both the inside and the outside, and proper thickness to it. Using the extrusion tool, you can only push in or out the faces and the backfaces are not maintained, so it really isn't accomplishing anything useful for that situation.
A workaround is to create a second cylinder, slightly larger and flip the normals. But it takes more time than I would like, compared to having a "thickness" or "maintain backfaces" check mark in the Extrude Faces dialog. Would also be helpful for turning planes into walls or floors with actual thickness.